1. Field of the Invention
The present invention relates to a discharge lamp lighting apparatus for lighting a discharge lamp such as, for example, a fluorescent lamp.
2. Description of the Related Art
Heretofore, discharge lamp lighting apparatuses as shown in FIGS. 1, 2 and 3 are known.
A discharge lamp lighting apparatus shown in FIG. 1 has a transformer 2 having a primary winding 2A and a secondary winding 2B magnetically coupled to the winding 2A. The primary winding 2A is connected to a commercial AC power source 1, and the secondary winding 2B is connected to a discharge lamp 3. In this apparatus, the self-inductance of the primary winding 2A and the mutual inductance of the primary and secondary windings 2A and 2B are operated as an impedance to the AC power source 1. Electric power is transmitted from the primary winding 2A side to the secondary winding 2B side by the mutual inductance. The number of turns of the secondary winding 2B is larger than that of the primary winding 2A so as to apply a high voltage to the discharge lamp 3. The impedance of the discharge lamp 3 abruptly drops from an infinite level by the start of discharging. At this time, the secondary winding 2B is operated as a choke coil for limiting the sine wave current which is to flow through the discharge lamp 3 to maintain the voltage applied to the discharge lamp 3 at a high level. Thus, the discharge of the discharge lamp 3 is continued.
The discharge lamp lighting apparatus shown in FIG. 2 has a half-wave voltage resonance type inverter. This inverter has a DC power supply circuit 4, a capacitor 5, a transformer 6, a resonance switch 7, a diode 8, a capacitor 9 and a choke coil 10.
The DC power supply circuit 4 has a filter, a rectifier, a smoothing capacitor, etc., to convert an AC voltage from the commercial AC power source 1 into a DC voltage. The capacitor 5 is connected in parallel with the primary winding 6A of the transformer 6 to form a resonance circuit. The resonance switch 7 is connected in series with the resonance circuit between the positive and negative terminals of the DC power supply circuit 4, and the diode 8 is connected in parallel with the resonance switch 7. The resonance switch 7 is constituted by a switching transistor and a control circuit for switching the transistor, for example. The secondary winding 6B of the transformer 6 is connected in series with the capacitor 9 and the choke coil 10 between both ends of the discharge lamp 3.
In this apparatus, when the resonance switch 7 is closed to electrically connect the resonance circuit to the DC power supply circuit 4, a current flows through the primary winding 6A. After the resonance switch 5 is opened, the primary winding 6A causes the current to continuously flow for a while and the capacitor 5 is charged by the current from the primary winding 6A. After the charging, the capacitor 5 causes the current to reversely flow through the primary winding 6A so as to keep the power supplied from the DC power supply circuit 4. The potential of the junction of the switch 7 and capacitor 5 tends to drop below that of the negative terminal of the DC power supply circuit 4 while the junction is electrically disconnected from the negative terminal. The diode 8 is set in a forward-biased state by the potential drop, and allows a current to flow from the negative terminal to the junction. Accordingly, the potential of the junction is maintained at a level not lower than that of the negative terminal. The resonance switch 7 is again closed while the current flows through the diode 8. When the resonance switch 7 is closed, the direction of the current flowing through the primary winding 6A is reversed with a delay of time. Thereafter, the above-described operation is again repeated.
Since the resonance switch 7 is closed while the potential difference between both ends thereof is set at substantially zero by the diode 8, it is possible to suppress the power loss in an LC resonance between the capacitor 5 and the primary winding 6A. Part of the resonance energy is transmitted from the primary winding 6A to the secondary winding 6B to allow an AC current to flow in the secondary winding 6B. This AC current is smoothed by the choke coil 10 to become a sine wave, and supplied to the discharge lamp 3. The capacitor 9 removes DC component from the current supplied to the discharge lamp 3.
The discharge lamp lighting apparatus shown in FIG. 3 has a half-bridge type inverter. This inverter has a DC power supply circuit 4, diodes 11 and 12, switches 13 and 14, capacitors 15 and 16, and a choke coil 17. The cathode terminal of the diode 11 is connected to the positive terminal of the DC power supply circuit 4, the anode terminal of the diode 11 is connected to the cathode terminal of the diode 12, and the anode terminal of the diode 12 is connected to the negative terminal of the DC power supply circuit 4. The switches 13 and 14 are respectively connected in parallel with the diodes 11 and 12. The capacitors 15 and 16 are connected in series between the positive and negative terminals of the DC power supply circuit 4, and a discharge lamp 3 is connected in series with the choke coil 17 between the junction of the capacitors 15 and 16 and the junction of the diodes 11 and 12.
In this apparatus, the switches 13 and 14 are alternatively closed. When the switch 13 is closed and the switch 14 is opened, a current flows from the positive terminal of the DC power supply circuit 4 through the switch 13, the choke coil 17, and the discharge lamp 3 to the capacitors 15 and 16. The capacitors 15 and 16 are changed by the current. Then, the switch 13 is opened. At this time, a current flows from the negative terminal of the DC power supply circuit 4 through the diode 12, the choke coil 17, and the discharge lamp 3 to the capacitors 15 and 16, since the choke coil 17 operates to allow the current to continue flowing therethrough. The switch 14 is closed when the potential difference between both ends of the switch 14 becomes zero by the current flowing through the diode 12. At this time, the capacitors 15 and 16 discharge the stored charges as a current, which flows through the discharge lamp 3, the choke coil 17, and the diode 12 to the negative terminal of the DC power supply circuit 4. Then, the switch 14 is opened. At this time, a current flows from the capacitors 15 and 16 through the discharge lamp 3, the choke coil 17 and the diode 11 to the positive terminal of the DC power supply circuit 4, since the choke coil 17 operates to allow the current to continue flowing therethrough. The above-described switching operation is repeated in the subsequent cycles. Similarly to the apparatus shown in FIG. 2, this discharge lamp lighting apparatus can suppress the power loss in an LC resonance between the capacitors 15 and 16 and the choke coil 17.
However, the configurations of the discharge lamp lighting apparatuses shown in FIGS. 1 to 3 do not allow for reductions in size and weight.
Since the discharge lamp lighting apparatus shown in FIG. 1 employs the commercial AC power source 1 at a frequency of about 50 Hz, a transformer having a large inductance is needed to obtain an output impedance capable of maintaining the voltage applied to the discharge lamp 3 at a high level required for continuing the discharge after the discharge lamp is ignited. The size and weight of such a transformer are substantially proportional to the inductance thereof. The inductance of the transformer can be reduced, for example, by increasing the frequency of the power source. However, this alternation is restricted since the discharge lamp must be lit without bearing harmful radiations, which are controlled by regulations.
When a fluorescent lamp is AC-lit as the discharge lamp, it is desirable to set the lighting frequency (=the frequency of the power source voltage) within a range of 10 kHz to 50 kHz so as to comply with the regulations. If the lighting frequency is set outside of this range, infrared remote controllers and radio receivers, for example, can not operate properly due to interference from the radio and infrared noises radiated from the lamp. Therefore, the frequency of the power source voltage cannot be increased over 50 kHz. The size and weight of the transformer cannot be satisfactorily reduced with the alteration of the frequency of this degree.
In the discharge lamp lighting apparatus shown in FIGS. 2 and 3, the lighting frequency of the discharge lamp is determined according to the resonance frequency f=1/(2.pi..sqroot.LC), of the inverter. Since the resonance circuit is composed in combination of the coil, the transformer and the capacitor, the size and weight can be reduced as compared with the case of preparing the sole transformer as in the apparatus shown in FIG. 1. However, if the resonance frequency of the inverter is increased higher than 50 kHz, the harmful infrared and radio noises are radiated from the discharge lamp similarly to the apparatus shown in FIG. 1. Thus, it is difficult to form the resonance circuit by using circuit parts of smaller sizes and lighter weights.
FIGS. 4 and 5 are plan and side views of an example of mounting parts of a discharge lamp lighting apparatus shown in FIG. 3. The circuit parts of the example have the necessary lowest withstand voltages and element values necessary to set the resonance frequency to 50 kHz or less, and are mounted on a circuit board 18 of phenol resin substantially as groups of functional modules. Transistors 24 (i.e., switches 13 and 14) are fixed to a heat sink plate 23, and disposed at the center of the circuit board together with a current transformer 25, a control part 26, and preheating capacitors 27. The current transformer 25 is used to supply base currents to the transistors 24 for self-excited oscillation. A fuse 19 and a DC power supply circuit 4 (i.e., filter elements 20, a rectifier 21, a smoothing capacitor 22) are disposed on one side of the circuit board 18. Further, resonance capacitors 15 and 16 and a choke coil 17 are disposed on the other side of the circuit board 18. An insulation film 29 is adhered to the rear surface of the circuit board 18 opposed to the front surface in which the circuit parts exist, and the circuit board 18 is placed in an aluminum case 28.
As shown in FIGS. 4 and 5, the choke coil 17, the current transformer 25 and the resonance capacitors 15 and 16 are larger than the other circuit parts. Accordingly, when all the circuit parts are mounted on the circuit board 18, the heights are irregular. The height of the aluminum case 28 must be determined in coincidence with the largest part. This make it difficult to reduce the thickness of the entire discharge lamp lighting apparatus.